1. Field of the Invention
The present invention relates to a gear reducer for obtaining rotation of an output shaft through a predetermined angle of rotation in either direction by reducing forward and reverse rotation of an input shaft, and more particularly to a gear reducer for rotation which is applicable to, for instance, a reclining mechanism of an automobile seat, a legless chair, or a bed, or an articulating mechanism of an advertising robot whose arm, head or the like oscillates.
2. Description of the Related Art
Conventionally, as a gear reducer of this type, a gear reducer for use in an automobile power seat is disclosed in, for example, Japanese Utility Model Unexamined Publication No. 25055/1991.
This reducer is arranged such that an intermediate shaft having both a worm wheel meshing with a drive-side worm and a worm meshing with a driven-side worm wheel is provided in a worm gear reducing mechanism for effecting speed reduction by means of two or more sets of worm gears.
Japanese Utility Model Unexamined Publication No. 36146/1992 discloses a gear reducer which is similarly used for driving an automobile power seat. This gear reducer is arranged such that an internal gear 80 and an external gear 81, which are disposed eccentrically with each other in the radially inward and outward directions and are provided with different number of teeth, are made to mesh with each other. Meanwhile, as a drive shaft is rotated, either one of these gears 80 and 81 is made to rotate eccentrically by means of an eccentric cam 84, so that either one of the gears 80 and 81 rotates by the difference in the number of their teeth with respect to one revolution of the eccentric cam 84.
This Japanese Utility Model Unexamined Publication No. 36146/1992 also proposes a technique in which a transmission intermediate member 86 capable of undergoing resilient deformation and having a multiplicity of reinforcing members 85 is provided in a meshing portion between the internal gear 80 and the external gear 81.
However, the structure disclosed in the above-described Japanese Utility Model Unexamined Publication No. 25055/1991 has a drawback in that the efficiency drops to a very low level since two or more sets of worm gears, which are inherently low in efficiency, are combined in multiple stages. For this reason, in order to produce (permit) a large torque, it is necessary to increase, for instance, the diameter of a worm shaft, with the result that dimensional restrictions or weightwise restrictions in the specification of worms for securing rigidity become large. In some cases, a reinforcing member or the like for firmly supporting the worm shaft is required.
In addition, since the number of shaft axes becomes three or more, it is difficult to maintain high assembly accuracy, and the structure of a casing and the like becomes complicated.
On the other hand, with the Japanese Utility Model Unexamined Publication No. 36146/1992, component parts which are difficult to be formed as a unit are large in number, so that the assembly is difficult. In addition, there has been a major problem in that an output cannot be obtained by concentric rotation, as shown at (A) to (C) in FIG. 7. Namely, since an output member 88 is directly integrated with the external gear 81 which eccentrically rotates, not only the rotation of the external gear 81 but also the swaying thereof around the eccentric cam are outputted together, making it impossible to obtain a reciprocating motion on a single arc around a specific axis. Hence, it has been inconvenient to use it as a gear reducer of this type.
Moreover, since this structure is not provided with a so-called reverse rotation preventing mechanism, if a strong load is applied from an output side, each gear rotates as a result. Consequently, each time a strong load is applied, the position of a backrest moves slightly, so that there has been a problem which cannot be ignored as a reducer for rotation of this type.